Electrically conductive, thermosetting elastomeric material and uses therefor

ABSTRACT

An electrically conductive, thermosetting elastomeric composition is provided. The composition may comprise: an initially substantially non-electrically conductive, thermosetting base polymer; a particulate filler comprising electrically conductive particles; and an electrically conductive polymer additive. The non-electrically conductive, thermosetting base polymer, the particulate filler and the electrically conductive polymer additive are mixed substantially macroscopically homogeneously.

RELATED APPLICATIONS

This application claims the benefit of the priority of U.S. applicationNo. 61/113,998 filed 12 Nov. 2008 which is hereby incorporated herein byreference.

TECHNICAL FIELD

This invention relates generally to thermosetting polymers. Particularembodiments provide electrically conductive, thermosetting elastomericcompositions, uses therefor and methods for fabricating structurescomprising same.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than restrictive.

FIGS. 1A, 1B and 1C are photographs which illustrate the flexibility ofa sample structure fabricated from an electrically conductive,thermosetting elastomeric composition according to a particular exampleembodiment

FIG. 2 is a plot illustrating the AC electrical properties (frequencyresponse) of an electrically conductive, thermosetting elastomericmaterial according to a particular example embodiment and comparing thefrequency response to that of copper.

FIG. 3 shows a number of plots illustrating the stress-strain curves offour structures fabricated from corresponding exemplary electricallyconductive, thermosetting elastomeric compositions according toparticular embodiments of the invention.

FIG. 4 is a plot illustrating the correlation between particulate fillerconcentration (by weight) and volume resistivity of a structurefabricated from an electrically conductive, thermosetting elastomericcomposition according to an exemplary embodiment of the invention.

FIG. 5 is a plot illustrating the change in volume resistivity as afunction of the elongation for a particular sample of an electricallyconductive, thermosetting elastomeric material according to a particularembodiment of the invention.

FIG. 6 is a diagram showing a portion of a garment constructed usingstructures fabricated from electrically conductive, thermosettingelastomeric compositions according to particular embodiments of theinvention and suitable for breast cancer detection via ElectricalImpedance Scanning (EIS).

FIG. 7A is a cross-sectional diagram showing a micromolding process forfabrication of a tubular structure comprising electrically conductive,thermosetting elastomeric compositions according to particularembodiments of the invention on a non-electrically conductive polymersubstrate which may be used for lab-on-chip applications.

FIGS. 7B and 7C are micrographs showing example structures formed usingmicromolding processes similar to that of FIG. 7A.

FIG. 8 is a plot illustrating the conductivity of electricallyconductive, thermosetting elastomeric compositions versus a percentageweight of multi-walled carbon nanotube particulate filler.

FIGS. 9A-9C are scanning electron microscope (SEM) micrographs showingsurfaces of samples of electrically conducting, thermosetting elasomericmaterials according to particular example embodiments at various levelsof magnification.

FIG. 10A is an optical micrograph displaying micro-electrodes of variousdimensions fabricated using an electrically conductive, thermosettingelastomeric composition according to a particular embodiment of theinvention on a non-conductive PDMS substrate. FIG. 10B is an opticalmicrograph showing the FIG. 10A micro-electrodes and the PDMS substratein a twisted state to demonstrate their flexibility.

FIG. 11A schematically depicts an example method for fabricating amicromold which may be used to fabricate structures using electricallyconductive, thermosetting elastomeric compositions according toparticular embodiments of the invention. FIG. 11B schematically depictsan example method for fabricating an electrically conductive,thermosetting elastomeric structure using the FIG. 11A micromold.

FIGS. 12A-12E are micrographs showing an example flexible PCB-likecircuit board fabricated using a micromolding technique of the typedescribed herein and comprising an electrically conductive,thermosetting elastomeric composition of the type described herein.

DESCRIPTION

Throughout the following description specific details are set forth inorder to provide a more thorough understanding to persons skilled in theart. However, well known elements may not have been shown or describedin detail to avoid unnecessarily obscuring the disclosure. Accordingly,the description and drawings are to be regarded in an illustrative,rather than a restrictive, sense.

Aspects of the invention provide electrically conductive, thermosettingelastomeric compositions. In particular embodiments, the electricallyconductive, thermosetting elastomeric compositions comprise suitablecombinations of: an initially substantially non-electrically conductive,thermosetting base polymer; a conductive particulate filler (e.g.microparticles or nanoparticles); and a conductive polymer additive. Thenon-conductive thermosetting base polymer, conductive particulate fillerand conductive polymer additive may be mixed substantiallyhomogeneously. In some embodiments, the composition may further comprisea curing agent, a diluting agent or both.

In some embodiments, the non-conductive thermosetting base polymercomprises one or more of: a silicone-based elastomer and apolyurethane-based elastomer. In some embodiments, the non-conductivethermosetting base polymer comprises one or more of: PDMS, Sealant 734™and polyurethane-based elastomer. In some embodiments, thenon-conductive thermosetting base polymer comprises contact cement. Insome embodiments, the particulate filler comprises one or more of:metal-based microparticles, metal-based nanoparticles, carbon-basedmicroparticles and carbon-based nanoparticles. In some embodiments, themetal-based microparticles or metal-based nanoparticles comprise silvermicroparticles or silver nanoparticles. In some embodiments, an aspectratio of a maximum dimension to a minimum dimension of the metal-basedmicroparticles or metal-based nanoparticles is greater than or equal to50:1. In some embodiments, an aspect ratio of a maximum dimension to aminimum dimension of the carbon-based microparticles or carbon-basednanoparticles is greater than or equal to 250:1. In some embodiments,the conductive polymer additive comprises one or more of: PEDOT andPEDOT/PSS.

In particular embodiments having metal-based particulate fillermaterials, the concentration of particulate filler material in theresultant electrically conductive, thermosetting elastomeric compositionis less than 30% weight. In other embodiments having metal-basedparticulate filler materials, the concentration of particulate fillermaterial in the resultant electrically conductive, thermosettingelastomeric composition is less than 15% weight. In still otherembodiments having metal-based particulate filler materials, theconcentration of particulate filler material in the resultantelectrically conductive, thermosetting elastomeric composition is lessthan 10% weight. In particular embodiments having carbon-basedparticulate filler materials, the concentration of particulate fillermaterial in the resultant electrically conductive, thermosettingelastomeric composition is less than 5% weight. In other embodimentshaving carbon-based particulate filler materials, the concentration ofparticulate filler material in the resultant electrically conductive,thermosetting elastomeric composition is less than 2% weight. In stillother embodiments having carbon-based particulate filler materials, theconcentration of particulate filler material in the resultantelectrically conductive, thermosetting elastomeric composition is lessthan 1% weight. In yet other embodiments having carbon-based particulatefiller materials, the concentration of particulate filler material inthe resultant electrically conductive, thermosetting elastomericcomposition is less than 0.2% weight.

Particular embodiments provide methods for preparing electricallyconductive, thermosetting elastomeric structures comprising suitablecombinations of: an initially substantially non-electrically conductive,thermosetting base polymer; a conductive particulate filler (e.g.microparticles or nanoparticles); and a conductive polymer additive. Thenon-conductive thermosetting base polymer, conductive particulate fillerand conductive polymer additive may be mixed substantiallyhomogeneously. As will be described further herein, example embodimentsof electrically conductive, thermosetting compositions possess lowerresistivity, higher flexibility and stretchability, and improvedmicromoldability relative to the known art.

Particular embodiments, provide methods for micromolding structuresusing electrically conductive, thermosetting elastomeric compositionscomprising combinations of: an initially substantially non-electricallyconductive, thermosetting base polymer; a conductive particulate filler(e.g. microparticles or nanoparticles); and a conductive polymeradditive. In particular embodiments, such methods comprise softlithography techniques. Such methods may include fabricating micromoldsusing one or more iterations of UV exposure of a photoresist layerthrough a suitable mask and development of the resultant structure in amanner which impacts the exposed and unexposed photoresist in differentmanners.

Such methods may include one or more iterations of introducing theelectrically conductive, thermosetting elastomeric composition to a moldand curing the electrically conductive, thermosetting elastomericcomposition in the mold. Such methods may include layering one or morelayers of electrically conductive, thermosetting elastomeric compositionwith one or more layers of substantially non-electrically conductive,thermosetting elastomeric material. The non-electrically conductivematerial may comprise the base polymer used in the electricallyconductive composition. Layering one or more layers of electricallyconductive composition with one or more layers of one or more layers ofsubstantially non-electrically conductive material may involve one ormore iterations of introducing the electrically conductive compositionto a mold, curing the electrically conductive composition, introducingthe non-electrically conductive material to the mold and curing thenon-electrically conductive material.

Such methods may comprise: fabricating a first portion of a mold usingsoft lithography techniques; curing one or more first polymer layers inthe first portion of the mold; fabricating a second portion of the moldin a location against the first portion of the mold and the one or morefirst polymer layers using soft lithography techniques; and curing oneor more second polymer layers in the second portion of the mold; whereinat least one of the one or more first polymer layers and the one or moresecond polymer layers comprises the electrically conductive,thermosetting elastomeric composition. In some embodiments, at least oneof the one or more first polymer layers and at least one of the one ormore second polymer layers may comprise a non-electrically conductivethermosetting polymer.

Another aspect of the invention provides an electrically conductive,thermosetting elastomeric compositions comprising suitable combinationsof: an initially substantially non-electrically conductive,thermosetting base polymer; and a metal-based particulate filler (e.g.microparticles or nanoparticles) wherein an aspect ratio of a maximumdimension to a minimum dimension of the metal-based particulate filleris greater than or equal to 50:1. In some embodiments, this aspect ratiois greater than or equal to 250:1. The non-conductive thermosetting basepolymer and conductive particulate filler may be mixed substantiallyhomogeneously. In some embodiments, the composition further comprises aconductive polymer additive. In some embodiments, the composition mayfurther comprise a curing agent, a diluting agent or both. Methods offabrication, applications and structures fabricated from thiscomposition are also provided.

One aspect of the invention provides electrically conductive,thermosetting elastomeric compositions comprising suitable combinationsof: an initially substantially non-electrically conductive,thermosetting base polymer; a conductive particulate filler (e.g.microparticles or nanoparticles); and a conductive polymer additive. Thenon-conductive thermosetting base polymer, conductive particulate fillerand conductive polymer additive may be mixed substantiallyhomogeneously.

In particular embodiments, the initially non-electrically conductive,thermosetting base polymer comprises a thermosetting, initiallynon-electrically conductive elastomer. The thermosetting, initiallynon-electrically conductive base polymer may comprise a silicone-basedelastomer, such as, by way of non-limiting example: polydimethylsiloxane(PDMS); Sealant 734™ commercially available from Dow Corning (which maycomprise ethyltriacetoxysilane, methyltriacetoxysilane,decamethylcyclopentasiloxane, octamethylcyclotetrasiloxane, aceticanhydride, acetic acid and/or dipropyl methane); and/or othersilicone-based elastomers. Other non-limiting examples of suitableinitially non-electrically conductive, thermosetting base polymersinclude: polyurethane-based elastomers and contact cement (such as thebrand DAP™ Weldwood™ contact cement sold by DAP Products Inc. ofBaltimore, Md., USA, LePage™ Pres-Tite™ contact cement sold by HenkelCorporation, Canada or the like). In some embodiments, initiallynon-conductive, thermosetting contact cement may comprise toluene,solvent naptha (petroleum), magnesium oxide and 2-butanone or toluene,N-hexane, acetone, and aliphatic petroleum distillate. In otherembodiments, other initially non-electrically conductive elastomericmaterials may be used to provide the base material. Such initiallynon-electrically conductive, thermosetting base materials may compriseother materials, such as, by way of non-limiting example, one or moreof: ethyltriacetoxysilane, methyltriacetoxysilane,decamethylcyclopentasiloxane, octamethylcyclotetrasiloxane, aceticanhydride, acetic acid and dipropyl methane. The above-describedthermosetting, initially non-electrically conductive, base materialsrepresent non-limiting examples of suitable base materials. In someembodiments, other elastomeric materials could be used to provide thebase material, other thermosetting materials could be used to providethe base material and other initially non-electrically conductivematerials could be used to provide the base material.

In particular embodiments, the particulate filler material may comprisemetal-based microparticles and/or nanoparticles, carbon-basedmicroparticles and/or nanoparticles and/or combinations thereof. As usedherein, the term microparticles refers to particles having a minimumcross-sectional dimension greater than 1000 nm and the termnanoparticles refers to particles having a minimum cross-sectionaldimension less than or equal to 1000 nm. For example, a generallyspherical shaped silver particle having a minimum cross-sectionaldimension of 1200 nm would be a microparticle, but a rod-shaped silverparticle having a long dimension of 1200 nm, but a diameter (i.e. aminimum cross-sectional dimension) of 10 nm would be a nanoparticle.

Metal-based particulate filler materials may provide relatively highconductivity (as compared to carbon-based filler materials), whilecarbon-based particulate filler materials may provide relatively lightweight (as compared to metal-based filler materials). In currentlypreferred embodiments, the particulate filler material is conductive.Metal-based microparticles or nanoparticles may comprise pure metals,metallic alloys or other compounds containing metals. Examples ofmetal-based micro or nanoparticles which may be used to provideparticulate filler material include, without limitation, one or more of:silver, gold, platinum, copper, nickle, aluminum, zinc, molybdenum,cadmium, scandium, titanium, vanadium, chromium, manganese, iron,cobalt, yttrium, zirconium, niobium, tantalum, tungsten, lead and/ormetallic compounds or alloys containing these and/or any other metalliccompounds or alloys (e.g. indium tin oxide, terfenol-D, manganin,constantan or the like). Examples of carbon-based microparticles ornanoparticles include, without limitation, one or more of: carbon,graphene, graphite and/or fullerene.

In some embodiments having metal-based particulate fillers, theparticles which make up the particulate filler material may have maximumdimensions on the order of 1000 nm. In other embodiments havingmetal-based particulate fillers, this maximum dimension may be on theorder of 250 nm. In still other embodiments, this maximum dimension maybe on the order of 100 nm. Carbon-based particles (e.g. carbon nanotubesand the like) have been reported with dimensions as high as 1000 μm andmay soon be even higher. Particular embodiments of the invention mayincorporate such large carbon-based particulate filler materials. Insome embodiments having carbon-based particulate filler materials, themaximum dimensions may be on the order of 15 μm. In other embodimentshaving carbon-based particulate filler materials, the maximum dimensionsmay be on the order of 2 μm. In still other embodiments havingcarbon-based particulate filler materials, the maximum dimensions may beon the order of 500 nm. Without wishing to be bound by any particulartheory, it is currently believed that using smaller particles for theparticulate filler material may result in electrically conductive,thermosetting elastomeric compositions according to aspects of thisinvention that have relatively higher flexibility and lower resistivity.

In some embodiments, the particles which make up the particulate fillermaterial may have particular shapes, such as any one or more of: flakeshapes, fiber shapes, rod/wire shapes, single walled tube shapes (e.g.single walled carbon nano-tubes) and/or multi-walled tube shapes (e.g.multi-walled carbon nano-tubes). The shape of the particles may also bea factor in determining the flexibility and/or electrical conductivityof the resulting electrically conductive, thermosetting elastomericmaterial. In some embodiments having metal-based particulate fillermaterials, an aspect ratio of the maximum particle dimension to aminimum particle dimension of the particulate filler material is greaterthan or equal to 250:1. In other embodiments having metal-basedparticulate filler materials, this aspect ratio is greater than or equalto 50:1. In some embodiments having carbon-based particulate fillermaterials, this aspect ratio is greater than or equal to 10^(51:1). Inother embodiments having carbon-based particulate filler materials, thisaspect ratio is greater than or equal to 1000:1. In still otherembodiments having carbon-based particulate filler materials, thisaspect ratio is greater than or equal to 250:1. These maximum andminimum particle dimensions may refer to the nominal or average particledimensions of a quantity of particulate filler material rather than toevery individual particle of such filler material.

It is not necessary that the entirety of the filler material beconductive. In some embodiments, suitable filler particles are coatedwith conductive materials (e.g. otherwise non-conductive microparticlesor nanoparticles coated with a metallic layer, such as silver, gold,platinum or copper). In some embodiments, the particulate fillermaterial may comprise semiconductor-based microparticles ornanoparticles alone or in combination with metal-based and/orcarbon-based microparticles or nanoparticles. Non-limiting examples ofsuitable semiconductor-based microparticles or nanoparticles include:boron, silicon, germanium, selenium, cadmium selenide, cadmium sulfide,cadmium telluride, zinc selenide, zinc sulfide, zinc telluride, leadsulfide, lead telluride, tin sulfide, tin telluride, aluminiumphosphide, boron nitride, boron phosphide, boron arsenide, galliumantimonide, gallium arsenide, gallium nitride and the like.

In general, there is a trade-off related to the amount of particulatefiller material used. As explained in more detail below, the particularfilter concentration is desirably higher than a percolation thresholdconcentration, so as to achieve acceptably high electrical conductivity.However, excessive amounts of particulate filler material can counteractthe flexibility of the resultant material and structures fabricatedtherefrom and can add to the weight of the resultant material andstructures fabricated therefrom. The inventors have demonstrated thatthe addition of the conductive polymer additive to the composition ofthermosetting polymer and particulate filler material dramaticallyreduces the percolation threshold concentration and permitscorrespondingly lower concentrations of particulate filler materials. Inparticular embodiments having metal-based particulate filler materials,the concentration of particulate filler material in the resultantelectrically conductive, thermosetting elastomeric material is less than30% weight. In other embodiments having metal-based particulate fillermaterials, the concentration of particulate filler material in theresultant electrically conductive, thermosetting elastomeric material isless than 15% weight. In still other embodiments having metal-basedparticulate filler materials, the concentration of particulate fillermaterial in the resultant electrically conductive, thermosettingelastomeric material is less than 10% weight. In particular embodimentshaving carbon-based particulate filler materials, the concentration ofparticulate filler material in the resultant electrically conductive,thermosetting elastomeric material is less than 5% weight. In otherembodiments having carbon-based particulate filler materials, theconcentration of particulate filler material in the resultantelectrically conductive, thermosetting elastomeric material is less than2% weight.

In particular embodiments, the conductive polymer additive may compriseone or more conductive polymers including but not limited to:

-   -   poly(3,4-ethylenedioxythiophene) (PEDOT);    -   PEDOT doped with poly(styrenesulfonate) (PEDOT/PSS);    -   polyaniline;    -   poly(pyrrole);    -   poly(acetylene);    -   poly(thiophene);    -   poly(p-phenylene sulfide);    -   poly(para-phenylene vinylene) (PPV);    -   polyindole;    -   polypyrene;    -   polycarbazole;    -   polyazulene;    -   polyazepine;    -   polynaphthalene;    -   other conjugated polymers;    -   derivates of these materials; and/or    -   the like.        In some embodiments, the conductive polymer may comprise the        monomer ethylenedioxythiophene (EDOT) which may be subsequently        polymerized to provide PEDOT. This polymerization of EDOT to        provide PEDOT may carried out electrochemically or with an        oxidizing agent such as an iron(III) salt, for example. Families        of suitable conductive polymer products containing EDOT, PEDOT        and/or PEDOT/PSS are marketed by HC Stark under the tradenames        Baytron™ FHC and Clevios™ and by Agfa under the tradename        Orgacon™. A suitable family of conductive polymer products        containing polyaniline is marketed by Fibron Technologies under        the tradename nPAC™. The above-listed conductive polymers        represent non-limiting examples of suitable conductive polymers        which may be used in conjunction with particular embodiments. In        other embodiments, other suitable conductive polymers may be        used.

In addition to an initially substantially non-conductive base polymer, aconductive particulate filler and a conductive polymer additive,particular embodiments make use of an optional diluting agent. By way ofnon-limiting example, a suitable diluting agent may comprise heptane,hexane or toluene. Particular embodiments may also employ a suitablecuring agent to promote curing of the electrically conductive,thermosetting elastomeric materials during the preparation process asdescribed below. In such embodiments, curing agents are preferablychosen based on their compatibility with corresponding thermosettingbase polymers. Different base polymers may react with different curingagents. Some base polymers, such as Sealant 734™, may not require anycuring agent. Curing agents, although not required, may optionally beused to speed up the curing process. Curing agents are also known in theart as crosslinking agents.

Another aspect of the invention provides an electrically conductive,thermosetting elastomeric compositions comprising suitable combinationsof: an initially substantially non-electrically conductive,thermosetting base polymer; and a metal-based particulate filler (e.g.metal-based microparticles or nanoparticles) wherein an aspect ratio ofa maximum dimension to a minimum dimension of the metal-basedparticulate filler is greater than or equal to 50:1. In someembodiments, this aspect ratio is greater than or equal to 250:1. Thenon-conductive, thermosetting base polymer and conductive particulatefiller may be mixed substantially homogeneously. The non-conductive,thermosetting base polymer may have features similar to thenon-conductive, thermosetting base polymer described above. Other thanfor being metal-based and its particulate aspect ratio, the particulatefiller material may have features similar to the particulate fillermaterial described above. In some embodiments, the composition furthercomprises a conductive polymer additive. In some embodiments, thecomposition may further comprise a curing agent, a diluting agent orboth.

Particular embodiments provide methods for preparing electricallyconductive thermosetting elastomeric materials. Such methods maygenerally comprise: mixing a non-conductive base polymer, a conductiveparticulate filler, a conductive polymer additive and, optionally, adiluting agent and/or curing agent together; and then thermally curingthe mixture. Mixing is desirable to achieve a composition that issubstantially homogeneous (at least on a macroscopic scale).Non-limiting exemplary methods for mixing the constituent materialsinclude manual stifling, mechanical mixing (e.g. shear mixing, ballmilling, magnetic mixing) and ultrasonic mixing. In some embodiments,where the composition comprises carbon-based particulate fillermaterials, the portions of the carbon-based particles can befunctionalized (e.g. using surfactants or the like). Functionalizationof carbon-based particles can have the effect of causing thecarbon-based particles to repel one another in a manner which promoteshomogeneity in the corresponding composition. Functionalization ofcarbon-based particles may occur in addition to or as an alternative toother methods of mixing the constituent materials.

In particular embodiments, thermally curing the mixture may occur attemperatures less than or equal to 300° C. In some embodiments,thermally curing the mixture may occur at temperatures less than 150° C.In still other embodiments, thermally curing the mixture may occur atroom temperature (e.g. at temperatures less than 25° C.). Heat may beused to promote relatively rapid thermal curing. By way of non-limitingexample, in particular embodiments where the thermosetting base polymercomprises Sealant 734™, thermal curing may occur over a number of hoursat room temperature, but heat can be applied to thermally cure themixture in a number of minutes. The application of heat may similarlydecrease the curing times for mixtures comprising other base polymers.Advantageously, the mixture of the constituent materials may bemoldable. Accordingly, some preparation methods further comprise thestep of molding or micromolding the mixture prior to or during curing toprovide a resultant conductive elastomeric structure having desiredshapes and/or other characteristics. Such micromolding techniques may bereferred to as soft lithography. More detailed example methods forpreparing particular conductive thermosetting elastomeric materials andmicromolding techniques are provided below.

Addition of the conductive particulate filler material and conductivepolymer additive to the initially non-conductive, thermosetting basepolymer in accordance with the compositions and methods described hereincan produce an elastomeric material that is highly electricallyconductive and flexible, light weight and uses a relatively small amountof particulate filler material to achieve percolation. With respect toelectrical conductivity, electrically conductive, thermosettingelastomeric compositions according to particular example embodimentscomprising metal-based particulate filler materials provide volumeresistivity less than 0.05Ω·m. In other embodiments comprisingmetal-based particulate filler materials, this volume resistivity isless than 0.025Ω·m. In still other embodiments comprising metal-basedparticulate filler materials, this volume resistivity is less than0.01Ω·m. Particular example embodiments comprising carbon-basedparticulate filler materials provide volume resistivities less than25×10³Ω·m.

With respect to flexibility, example embodiments have exhibited theability to be twisted, folded over double and stretched to a high degreewithout substantial degradation of their conductivities. Thisflexibility is shown in FIGS. 1A, 1B and 1C which respectively showphotographs of a sample structure fabricated from an electricallyconductive, thermosetting elastomeric composition according toparticular example embodiment. The particular sample structure shown inFIGS. 1A, 1B and 1C has dimensions on the order of 2 mm×600 μm×500 μm atnormal temperature and pressure (NTP) conditions. As used in thisdescription, NTP conditions comprise temperatures of about 20° C.±2° C.and atmospheric pressures of about 1 atm±0.1 atm. FIG. 1A shows thesample at NTP conditions in a non-stressed state. FIG. 1B shows the samesample folded over double and FIG. 1C shows the same sample undertorsion (stretched and twisted). The sample material shown in FIGS. 1A,1B and 1C was prepared using an initially non-conductive base elastomerof Sealant 734™ and a conductive polymer additive of PEDOT at a 1:1weight ratio and a particulate filler comprising generally sphericalsilver nanoparticles having average dimensions of 80-500 nm. Theconcentration of silver nanoparticles in the sample material of FIGS.1A, 1B and 1C was 35% (by weight). The resistivity of the sample used inFIGS. 1A, 1B and 1C was on the order of 1.25×10⁻³Ω·m.

Structures fabricated from electrically conductive, thermosettingelastomeric compositions according to particular embodiments are capableof stretching up to 20% above their NTP dimensions without substantialincrease in resistivity. In other embodiments, structures fabricatedfrom electrically conductive, thermosetting elastomeric compositions arecapable of stretching up to 50% above their NTP dimensions withoutsubstantial increase in resistivity. In some embodiments, structuresfabricated from electrically conductive, thermosetting elastomericmaterials are capable of stretching up to 20% above their NTP dimensionswhile maintaining resistivities less than 10Ω·m. In some embodiments,electrically conductive, thermosetting elastomeric materials are capableof stretching up to 20% above their NTP dimensions while maintainingresistivities less than 5Ω·m. The degree to which resistivity isaffected by stretching is described in more particular detail below (seeFIG. 5).

Table 1 below shows experimental data for the volume resistivity andflexibility (measured as a percentage of stretching over the material'sambient dimension at NTP conditions) for compositions according tovarious example embodiments of the invention. In each of the Table 1examples, the non-conductive base polymer and PEDOT conductive polymeradditive were mixed in a 1:1 ratio with a sufficient amount ofparticulate filler material to be over the percolation threshold (thepercolation threshold is described in more detail below). The stretchingpercentage shown in Table 1 corresponds to the percentage stretchingwhere the sample exhibits structural damage or a rapid increase inresistivity. The Table 1 resistivity was measured at NTP conditions.

TABLE 1 Volume Resistivity and Stretching Percentage of VariousCompositions Stretch- # Composition Volume Resistivity ing % 1 elastomer(Sealant 734 ™) and 22 × 10³ Ω · m 25% PEDOT in a weight ratio of 1:1with carbon black particulate filler (average dimensions on the order ofhundreds of microns) 2 elastomer (Sealant 734 ™), and 15 × 10³ Ω · m 25%PEDOT in a weight ratio of 1:1 with graphite particulate filler 3elastomer (Sealant 734 ™) and 24 × 10⁻³ Ω · m 22% PEDOT in a weightratio of 1:1 with silver flakes (average volume 0.9-9 μm³) particulatefiller 4 elastomer (Sealant 734 ™) and 1.25 × 10⁻³ Ω · m 20% PEDOT in aweight ratio of 1:1 with silver nanoparticles (average dimension 80-500nm) particulate filler 5 contact cement (LePage ™ Pres- 8 × 10⁻³ Ω · m25% Tite ™) and PEDOT in a weight ratio of 1:1 with silver flakes(average volume 0.9-9 μm³) particulate filler 6 contact cement (LePage ™Pres- 1 × 10⁻³ Ω · m 22% Tite ™) and PEDOT in a weight ratio of 1:1 withsilver nanoparticles (average dimension 80-500 nm) particulate filler 7elastomer (Sealant 734 ™) and 20 × 10⁻³ Ω · m 80% PEDOT in a weightratio of 1:1 with silver nanoparticles (average dimension 80-500 nm) andsilver flakes (average volume 0.9-9 μm³) particulate filler

The particular brand of contact cement used in the Table 1 experimentswas LePage™ Pres-Tite™ contact cement sold by Henkel Corporation Canada,which may comprise solvent naptha (petroleum), magnesium oxide,2-butanone and toluene. The elastomer used in the Table 1 experimentswas Sealant 734™ which may comprise ethyltriacetoxysilane,methyltriacetoxysilane, decamethylcyclopentasiloxane,octamethylcyclotetrasiloxane, acetic anhydride, acetic acid and/ordipropyl methane. The Table 1 samples were prepared by ultrasonicallymixing the initially non-conductive base polymer (e.g. contact cement orSealant 734™ elastomer) with a diluting agent for approximately 30-60seconds. The conductive polymer (Baytron™ FHC (PEDOT)) was used for allthe Table 1 experiments) and particulate filler material(s) were thenadded and ultrasonically mixed for 60-90 seconds. The resultant mixtureswere then poured into molds having dimensions of 25 mm×25 mm×2 mm andallowed to cure (e.g. by leaving the compositions to cure under NTPconditions or by heating to compositions at temperatures less than about120° C.).

Electrically conductive, thermosetting elastomeric compositionsaccording to particular embodiments of the invention exhibit lowresistivity over a wide range of frequencies. FIG. 2 shows a plot of theAC frequency response (from 0-8 GHz) at NTP conditions of anelectrically conductive thermosetting elastomeric material according toa particular embodiment (Table 1, composition #4), together with a plotof the AC frequency response of copper. It can be seen that thefrequency response of the sample electrically conductive, thermosettingelastomeric material is comparable to that of copper.

FIG. 3 shows a number of plots illustrating the stress-strain curves offour exemplary electrically conductive, thermosetting elastomeric samplematerials according to particular embodiments of the invention. Thesestress-strain curves show the relationship between the stress applied toa sample (measured as the force used to stretch the sample) and thestrain (measured as the elongation of the sample as a percentage of itslength under NTP conditions). The particular sample materials shown inthe FIG. 3 plots comprise:

-   -   plot #1-elastomer (Sealant 734™) and PEDOT mixed in a weight        ratio of 1:1 with approximately 30% weight of generally        spherical silver nanoparticles with average dimensions of 80-500        nm;    -   plot #2-elastomer (Sealant 734™) and PEDOT mixed in a weight        ratio of 1:1 with approximately 15% weight of generally        spherical silver nanoparticles with average dimensions of 80-500        nm;    -   plot #3-polyurethane and PEDOT mixed in a weight ratio of 1:1        with approximately 30% weight of generally spherical silver        nanoparticles with average dimensions of 80-500 nm; and    -   plot #4-polyurethane and PEDOT mixed in a weight ratio of 1:1        with approximately 30% weight of generally spherical silver        nanoparticles with average dimensions of 80-500 nm.        As can be seen in FIG. 3, samples using the elastomer (Sealant        734™) base material (e.g. plot #2) can be stretched up to 40%        above their NTP dimensions and samples using the polyurethane        base material (e.g. plots #3 and #4) can be stretched up to 120%        above their NTP dimensions.

FIG. 4 is a graph illustrating the change in direct current volumeresistivity of structures fabricated from electrically conductive,thermosetting elastomeric compositions according to particularembodiments as a function of the concentration of conductive particulatefiller. The particular composition used to generate the FIG. 4 plotcomprised a non-conductive base polymer of Sealant 734™ and a conductivepolymer additive of PEDOT mixed in a 1:1 weight ratio with variousconcentrations of 80-500 nm (average dimension) generally sphericalsilver nanoparticle filler (measured as weight percentage). The datapoints used in the FIG. 4 plot are reproduced in Table 2 below. WhileFIG. 4 and the associated Table 2 data points are data for particularconstituent materials (e.g. Sealant 734, PEDOT and generally sphericalsilver nanoparticles), the shape and general characteristics of the FIG.4 resistivity curve are representative of similarly shaped resistivitycurves for compositions comprising other combinations of constituentmaterials. In the particular case of the FIG. 4 sample, it may beobserved that for concentrations of up to 8% by weight of particulatefiller, the resistivity is relatively high (e.g. on the order of 10⁴Ω·mor higher). However, with higher particulate filler concentrations, theresistivity decreases rapidly and remains low (e.g. less than100Ω·m)—i.e. there is a stark transition between the relativelynon-conductive state and the conductive state which depends on theconcentration of particulate filler material. The concentration ofparticulate filler material required for insulator-conductor transitionmay be referred to as the threshold concentration or percolationthreshold. The percolation threshold is approximately 8% weight in theillustrated example of FIG. 4. For the particular case of the FIG. 4constituent materials, a significantly low resistivity of 11Ω·m isachieved at concentrations of silver nanoparticles as low as 12% weight.

TABLE 2 Resistivity v. % Weight of Particulate Filler (FIG. 4)Resistivity (Ω · m) % Weight 10⁵  5 5 × 10⁴ 8 11  12 8 15 5 20 3 25 2 311 39   0.8 40   0.6 44   0.4 48   0.2 52   0.09 56

In some embodiments, it is desirable to have a relatively lowpercolation threshold (i.e. low resistivity at a relatively lowconcentration of particulate filler). Benefits of a relatively lowpercolation threshold include: decreased weight of structures fabricatedfrom the electrically conductive, thermosetting elastomeric composition(which is particularly advantageous in weight sensitive applicationssuch as automotive and aerospace applications); increased flexibility ofthe resulting electrically conductive, thermosetting elastomericmaterial, as flexibility can be reduced with the addition of largeamounts of particulate filler materials; and decreased cost, asparticulate filler materials can be expensive. Increased flexibilityallows the material to be used in applications requiring greaterflexibility and strength such as, by way of non-limiting example,conductive garments for electrical measurement, RF patch antennas,microelectronics and flexible EMI shielding.

The resistivity versus particulate filler concentration curves ofcompositions without conductive polymer additive (i.e. comprising aninitially non-conductive, thermosetting base polymer and conductiveparticulate filler material) look similar to that of FIG. 4. However,without the conductive polymer additive, the percolation threshold ofsuch materials is significantly higher than the 8% weight exhibited forthe FIG. 4 composition. While different compositions (which incorporatean initially non-conductive, thermosetting base polymer, conductiveparticulate filler material and conductive polymer additive) will havedifferent percolation thresholds, the inventors have demonstrated thatthese percolation thresholds will be significantly less than they wouldbe for similar compositions without the conductive polymer additive.

In compositions according to particular embodiments, the weight ratio ofinitially non-conductive, thermosetting base polymer to conductivepolymer additive is in a range of 0.9:1 to 1.1:1. In some embodiments,this ratio is in a range of 0.75:1 to 1.25:1. In other embodiments, thisratio is in a range of 0.5:1 to 1.5:1. It is expected that compositionscomprising relatively larger concentrations of conductive polymeradditive will exhibit relatively lower percolation thresholds.

FIG. 5 is a plot illustrating the change in volume resistivity (in Ω·m)as a function of the amount of stretching (in mm relative to the NTPdimensions) of a long dimension of a 25 mm×25 mm×2 mm sample structureof an electrically conductive, thermosetting elastomeric material formedaccording to Table 1 composition #4 with a particulate fillerconcentration of approximately 40% by weight Ag nanoparticles havingaverage dimensions of 80-500 nm. The data points used in the FIG. 5 plotare reproduced in Table 3 below. FIG. 5 shows that as the sample isstretched, its resistivity increases, but after stretching by 7 mm (from25 mm to 32 mm—i.e. a relative stretch of approximately 28% of itsoriginal dimension), the resistivity of the sample structure remainsbelow approximately 5Ω·m. FIG. 5 shows that structures fabricated fromthe electrically conductive, thermosetting elastomeric materialsaccording to various embodiments of the invention may provide goodconductivity even after considerable deformation.

TABLE 3 Resistivity v. Stretching (FIG. 5) Stretch (mm) Resistivity (Ω ·m) 0 0.09 1 0.1 2 0.3 3 0.7 4 1.125 5 2.4 6 4.2 7 4.8

Without wishing to be bound by any theory, it is currently thought thatthe increased resistivity shown in FIG. 5 is attributable to theincreased spacing between the particulate filler particles that resultsfrom stretching. In effect, stretching decreases the volumeconcentration of particulate filler materials, thereby slightlydecreasing the overall conductivity of the material. The inventorstheorize that as long as stretching does not reduce the effective volumeconcentration of particulate filler material beyond a critical point atwhich continuity of particulate filler materials is lost, the overallsample will remain highly electrically conductive. The inventors expectthat compression of structures fabricated from the electricallyconductive, thermosetting elastomeric compositions of the type describedherein will cause a corresponding increase in conductivity.

FIGS. 9A and 9B show scanning electron microscope (SEM) micrographs ofan electrically conductive, thermosetting elastomer compositionaccording to a particular embodiment of the invention. The compositionused in the FIGS. 9A and 9B micrographs comprised: an initiallynon-conductive, thermosetting base polymer of Sealant 734™ mixed with aconductive polymer additive of PEDOT (at a 1:1 weigh ratio) with aparticulate filler comprising generally spherical Ag particles havingdimensions on the order of 80-500 nm and a concentration of about 50%weight. FIG. 9A is a micrograph at a magnification of approximately130×. FIG. 9B is a micrograph at a magnification of approximately25×10³×. FIG. 9C is a similar scanning electron microscope (SEM)micrograph of an electrically conductive, thermosetting elastomercomposition comprising an initially non-conductive, thermosetting basepolymer of PDMS mixed with a conductive polymer additive of PEDOT (at a1:1 weigh ratio) with a particulate filler comprising multiwalled carbonnanotubes a concentration of about 2% weight. The FIG. 9C micrograph isat a magnification of approximately 26.3×10³×. FIGS. 9A-9C demonstratethat structures formed from these compositions are relatively uniformand that the particulate filler material is reasonably homogeneouslydispersed in the composition.

FIG. 10A is an optical micrograph displaying micro-electrodes of variousdimensions fabricated using an electrically conductive, thermosettingelastomeric composition according to exemplary embodiments of theinvention fabricated on a non-conductive PDMS substrate. FIG. 10B is anoptical micrograph showing the FIG. 10A micro-electrodes and the PDMSsubstrate in a twisted state to demonstrate their flexibility. Themicro-electrodes in the FIGS. 10A and 10B micrographs have heights of 30μm, widths of 100 μm and lengths ranging from 1 mm-10 mm.

The thermosetting quality of the elastomeric conductive materialsprovided by the various embodiments of this invention make themparticularly amenable to being molded and micromolded. Micromoldingrefers to the molding of features measured on the micrometer tonanometer scale. In some embodiments, micromolding can involve featuresizes of less than 1000 μm. In some embodiments, micromolding caninvolve feature sizes of less than 1000 nm. Various techniques ofmicromolding are known in the art, such as for example, soft lithographytechniques which involve lithographically patterning a mold, introducingthe material to and curing the material in the mold and peeling thecured material out of the mold. Thermosetting elastomers are suitablefor molding because they are usually liquid or malleable prior to curing(allowing them to conform to any mold shape), but then transform into astronger solid form upon curing. Because the curing process is generallyirreversible, thermosetting elastomers do not soften on subsequentheating (as opposed to thermoplastic elastomers that may lose theirsolidity and shape when reheated). Other advantages of thermosettingelastomers relative to thermoplastics are that thermosetting elastomerstend to be stronger and more durable than thermoplastic elastomers andthermosetting elastomers tend to be easier to micromold thanthermoplastic elastomers.

FIG. 11A schematically depicts an example method 100 for fabricating amicromold 150 which may subsequently be used to fabricate structuresusing electrically conductive, thermosetting elastomeric compositionsaccording to particular embodiments of the invention. Method 100involves techniques which may be referred to as soft lithography.

In the exemplary illustrated embodiment, method 100 commences at step110 which involves deposition of an adhesion layer 112 onto a substrate114. In the illustrated embodiment, substrate 114 comprises glass,although other suitable materials could be used for substrate 114. Inthe illustrated embodiment, adhesion layer 112 comprises chromium whichadheres to both the glass of substrate 114 and the SU-8 of photoresist116, although this is not necessary and adhesion layer 112 couldcomprise other materials (e.g. other metals, other metal alloys orcompounds or other materials that could adhere to both substrate 114 andphotoresist 116). By way of non-limiting example adhesion layer 112 maybe applied to substrate 114 by sputtering, electron beam deposition orother suitable vacuum deposition technique.

In step 120 of the exemplary method 100, a layer 116 of UV sensitivephotoresist material is coated atop metalization layer 112. Inparticular embodiments, UV sensitive photoresist 116 may comprise anepoxy based resin, such as SU-8 or the like, for example. In particularembodiments, photoresist material may be applied by spin coating, butother deposition techniques (e.g. casting) could be used. Suchdeposition techniques may depend on the type of photoresist materialused. Photoresist 116 may be applied at any suitable thickness which maydepend on the application to which the resultant mold is to be used. Inparticular embodiments, the thickness of photoresist 116 may be in arange of 50 μm-100 μm, although this is not necessary and photoresist116 may be applied in any suitable thickness. In step 120, the structure(e.g. including substrate 114, adhesion layer 112 and photoresist layer116) may be baked or otherwise heated (e.g. on a hotplate). The heatingtemperatures and durations may depend on the type of photoresistmaterial in photoresist layer 116 and on the type of heating apparatus.In particular embodiments, the structure may be heated on a hotplate attemperatures in a range of 75°-115° C. for a duration of 60-100 minutes.

In block 130 a patterned mask 118 is interposed between photoresist 116and a source of UV radiation 122. Mask 118 is patterned to provideregion(s) which are transparent to UV radiation and region(s) which areopaque to UV radiation. The result is that some regions 116A ofphotoresist 116 are exposed by the UV radiation and other regions 116Bof photoresist 116 remain unexposed. Exposure of regions 116A ofphotoresist 116 causes at least partial cross-linking of the molecularchains of photoresist 116 in exposed regions 116A or otherwise causesphotoresist 116 to cure or harden in exposed regions 116A. Regions 116Bof photoresist 116 that are not exposed to UV radiation remain uncured.In block 130, the structure (e.g. including substrate 114, adhesionlayer 112 and photoresist layer 116) may be again baked or otherwiseheated (e.g. on a hotplate). This optional block 130 post-exposureheating process may be used to ensure that photoresist is fullycross-linked in exposed regions 116A. The heating temperatures anddurations may depend on the type of photoresist material in photoresistlayer 116 and on the type of heating apparatus employed. In particularembodiments, the structure may be heated on a hotplate at temperaturesin a range of 50°-70° C. for a duration of 50-80 minutes.

In step 140, the structure is introduced to a solvent bath or solvent isotherwise applied to the structure in a development step to remove theuncured photoresist 116 from regions 116B. In step 140, the structuremay also be rinsed with isopropyl alcohol and/or de-ionized water toclean the structure. At the conclusion of step 140, the structure isready for use as mold 150 for fabricating structures using electricallyconductive, thermosetting elastomeric compositions according to variousembodiments of the invention.

Mold fabrication process 100 of FIG. 11A represents one particularexemplary mold fabrication process. In other embodiments, otherprocesses may be used to form molds. Such processes may include, by wayof non-limiting example, chemical and/or laser etching processes,mechanical machining processes and/or the like. In other embodiments,molds may be fabricated from other materials. Such materials maycomprise any suitable material, including, by way of non-limitingexample, metals, glass, suitable plastics, epoxy based resins, silicone,silicon, clay, granite, mica and the like. It is desirable that thematerials used to form the mold are not overly sensitive to theconstituent materials used in the electrically conductive, thermosettingelastomeric composition. For example, where the electrically conductive,thermosetting elastomeric composition uses an organic solvent as adiluting agent, it is desirable that the materials used to form the moldnot be overly sensitive to this organic solvent.

FIG. 11B schematically depicts an example method 200 for fabricating anelectrically conductive, thermosetting elastomeric structure using theFIG. 11A micromold 150. Method 200 involves techniques which may bereferred to as soft lithography. As will be described in more detailbelow, method 200 involves fabrication of structures having adjacentlayers of conductive and non-conductive elastomeric materials. Method200 commences in step 210 which involves fabricating the micromold 150.Micromold 150 may be fabricated according method 100 described above,but may also be fabricated or otherwise provided according to anysuitable process.

In step 220, an electrically conductive, thermosetting elastomericcomposition 222 (referred to as conductive nanocomposite 222 for thepurposes of describing method 200) is prepared and poured or otherwiseintroduced into mold 150. Typically, excess conductive nanocomposite 222is introduced as shown in step 220, but this is not necessary. As partof step 220, conductive nanocomposite 222 may be de-gassed prior tocuring to remove air bubbles. Such de-gassing may involve placingconductive nanocomposite 222 (along with mold 150) in a vacuum chamberfor a period of time (e.g. 10-45 minutes). The amount of time spentde-gassing conductive nanocomposite 222 may depend on the amount ofconductive nanocomposite 222 present and the properties of the vacuumchamber.

In step 230, excess conductive nanocomposite 222 is removed from mold150. The removal of excess conductive nanocomposite 222 in step 230 mayinvolve a Damascene-like process. The removal of excess conductivenanocomposite 222 in step 230 may involve scraping excess conductivenanocomposite 222 from the surface of mold 150 using a suitably sharpinstrument such as a knife or the like. The step 230 removal ofconductive nanocomposite 222 may be done before and/or after conductivenanocomposite 222 is permitted to cure. During step 230, the structure(including mold 150 and conductive nanocomposite 222) may optionally beheated to promote curing of conductive nanocomposite 222 (e.g. to makethe curing of conductive nanocomposite 222 occur relatively quickly). Byway of non-limiting example, mold 150 and conductive nanocomposite 222may be baked on a hot plate at temperatures in a range of 50° C.-150° C.for times in a range of 15 minutes to 2 hours. The structure may beheated using other suitable heating implements or techniques. Theparticular temperatures and time ranges associated with the heatingprocess may depend on the heat application technique and the particularsof the thermosetting material that is used to provide conductivenanocomposite 222.

In step 240, a substrate layer 224 is applied atop mold 150 andconductive nanocomposite material 22. In particular embodiments, such asthe illustrated embodiment of FIG. 11B, substrate layer 224 may comprisethe non-conductive thermosetting base polymer used in the composition ofconductive nanocomposite 222. For example: where conductivenanocomposite 222 comprises a PDMS base polymer, substrate layer 224 maycomprise non-doped (and non-conductive) PDMS; or where conductivenanocomposite 222 comprises a Sealant 734™ base polymer, substrate layer224 may comprise non-doped (and non-conductive) Sealant 734™ For thisreason, substrate layer 224 may also be referred to as a non-conductivepolymer layer 224 or a non-conductive elastomer layer 224.

As part of step 240, substrate layer 224 may be de-gassed prior tocuring to remove air bubbles. Such de-gassing may involve placing thestructure in a vacuum chamber for a period of time (e.g. 10-45 minutes).The amount of time spent de-gassing substrate layer 224 may depend onthe amount of substrate layer 224 present and the properties of thevacuum chamber. During step 240, the structure (including mold 150,conductive nanocomposite 222 and substrate layer 224) may optionally beheated to promote curing of substrate layer 224 (e.g. to make the curingof substrate layer 224 occur relatively quickly). By way of non-limitingexample, the structure may be baked on a hot plate at temperatures in arange of 50° C.-150° C. for times in a range of 15 minutes to 2 hours.The structure may be heated using other suitable heating implements ortechniques. The particular temperatures and time ranges associated withthe heating process may depend on the heat application technique and theparticulars of the material used to provide substrate layer 224.

Step 250 involves peeling the hybrid structure 226 (comprising curedsubstrate layer 224 and cured conductive nanocomposite 222) from mold150.

It will be appreciated by those skilled in the art that the exemplaryprocesses for mold fabrication (method 100 of FIG. 11A) and forfabricating structures comprising electrically conductive, thermosettingelastomeric compositions (method 200 of FIG. 11B) represent relativelysimple processes for the purpose of illustrating various aspects of theinvention. In other embodiments, molds may be fabricated using methodssimilar to method 100 but which involve multiple repetition of masking,exposing, developing and/or other steps to create multi-layer molds.Similarly, in other embodiments, structures may be fabricated fromelectrically conductive, thermosetting elastomeric compositions usingmethods similar to method 200 but which involve repetitive applicationof multiple layers of conductive nanocomposite and/or non-conductivepolymer and/or repetition of other steps to fabricate desiredstructures. In this manner, layers of patterned, electricallyconductive, thermosetting elastomeric materials may be embedded betweennon-conductive layers of thermosetting elastomeric materials forparticular applications. In particular applications, vias or the likemay be patterned or otherwise provided to provide conductive contactbetween layers of electrically conductive, thermosetting elastomericmaterials that are otherwise separated by non-conductive materials. Suchvias may be filled with electrically conductive, thermosettingelastomeric materials.

In still other embodiments, fabrication of the mold and forming astructure in the mold may take place in interleaving steps. For example,a first mold layer may be formed; a first structure layer (comprising anelectrically conductive, thermosetting elastomeric composition layerand/or a non-conductive polymer layer) may then be introduced and curedin the first mold layer; a second mold layer may be formed against (e.g.on top of) the first mold layer and/or the first structure layer; and asecond structure layer (comprising an electrically conductive,thermosetting elastomeric composition layer and/or a non-conductivepolymer layer) may then be introduced and cured in the second moldlayer. This process of interleaving mold and structure fabrication stepsmay be repeated as desired to fabricate a desired structure.

Electrically conductive, thermosetting elastomeric materials accordingto various embodiments of this invention may have application in manyfields for which flexible and conductive moldable materials are desired.By way of non-limiting example, some potential applications of variousembodiments of the invention include:

-   -   flexible electrical interconnects (electrical lines) on a single        chip or for chip to chip connection;    -   integration of flexible conductive tracks for signal integration        and processing in microfluidic and/or lab-on-chip systems        (including wearable microfluidic devices and their associated        electronics);    -   micro-instrumentation, microelectronics, and        microelectromechanical systems;    -   all-polymer microsystems;    -   miniaturized health care instrumentation—e.g. shape conforming        (moldable) or deformable electrodes for impedance-based        cancer/tumor detection;    -   actuators based on extrinsic conducting polymers;    -   flexible tactile and pressure sensors;    -   bistable memory devices, in particular polymer based bistable        memory devices/switches;    -   RF and microwave applications such as tunable antennas, passive        microwave components and conductive gaskets;    -   automotive industry applications;    -   wearable electronics;    -   flexible resistors;    -   flexible micro-electrodes for flexible electronics, lab-on-chip        systems or the like; and    -   microheaters.

A number of particular exemplary and non-limiting applications are nowdescribed.

Light Weight Wiring for Transportation Applications

Copper cables currently contribute to approximately 7% of an aircraft'sweight and as much as one-third of the weight of a 15-ton satellite.Similarly, copper wiring may make up a significant percentage ofvehicular weight for other transportation applications (e.g. spacecraft,automobiles, watercraft). Light weight cables fabricated out of theelectrically conductive, thermosetting elastomeric materials describedherein weigh substantially less than copper cable (e.g. on the order of¼ of the weight of copper) and their AC and DC electricalcharacteristics are similar to those of bulk copper. Accordingly, cablesfabricated (e.g. by molding or otherwise) from the electricallyconductive, thermosetting elastomeric materials described herein maysubstantially reduce the weight currently contributed by copper cables.

Wearable Electronics

One example of a wearable electronic garment which may make use of theconductive elastomeric materials described herein is a flexible,electrode-embedded brassiere or the like incorporating breast-receivingcups comprising electrically conductive blastomeric material for breastcancer detection via Electrical Impedance Scanning (EIS). Such a garmentmay comprise an array of electrodes (formed from an electricallyconductive, thermosetting elastomeric material of a type describedherein) which may be embedded in or layered with an otherwisenon-electrically conductive material (e.g. a non-conductive,thermosetting flexible polymer base). Individual electrodes may beelectrically connected by routing wires fabricated from electricallyconductive, thermosetting elastomeric materials described herein.Fabrication of the garment may be based on micromolding techniques ofthe type described herein and/or any other suitable fabricationtechnique. Arrays of a thousand or more electrodes with electronicrouting can be realized via micromolding techniques. The garment can beused to locate an array of electrodes directly against a patient'sbreast to permit EIS in a garment that is flexible and thereforesignificantly more comfortable than current EIS technology.

FIG. 6 shows a schematic drawing of a breast cup of an EIS garmentfabricated according to a particular embodiment of the invention whichmay be used to implement EIS testing for breast cancer. The FIG. 6 cupincorporates electrodes and interconnection lines fabricated fromelectrically conductive, thermosetting elastomeric materials of thetypes described herein and fabricated from micromolding techniques ofthe type described herein.

Micro-Instrumentation for Polymer-Based Lab on a Chip Applications

Electrically conductive elements may be fabricated on polymer-basedsubstrates or other substrates using the electrically conductive,thermosetting elastomeric materials described herein. For the particularexample of microfluidic applications, electrodes may be shaped (e.g. bymicromolding according to the techniques described herein) to providebores (e.g. tubular shapes) and may thereby provide flexible fluidconduits (through their bores) which may also be electrically conductive(through the material itself). Using micromolding techniques, variousfluid carrying and electrically conductive structures could be connectedto one another.

FIG. 7A depicts a cross-sectional view of a micromold 300 forfabricating a tubular structure 310 using electrically conductive,thermosetting elastomeric material 312 of the type described herein on anon-conductive polymer substrate 314. Mold 300 may be fabricated from atechnique similar to mold fabrication technique 100 described above,except that mold 300 of the FIG. 7A embodiment comprises multiple levelsand may be fabricated using a multiple level mask and/or using multiplemasking, exposing and/or developing steps. Tubular structure 310 may befabricated using a technique similar to structure fabrication technique200 described above, wherein electrically conductive, thermosettingelastomeric material 312 is introduced into mold 300 and then cured andthen non-conductive polymer material 314 is applied overtop and thencured, so that the resultant structure 300 may be peeled from mold 300.

FIGS. 7B and 7C are micrographs of tubular structures fabricated usingmolds like mold 300 from an electrically conductive, thermosettingelastomeric composition of the type described herein. The particularcomposition used to provide the structures in FIGS. 7B and 7C comprisesa base polymer of PDMS and a conductive polymer additive of PEDOT in a1:1 weight ratio and a particulate filler of 80 nm-500 nm generallyspherical silver nanoparticles with a concentration of about 45% weight.The FIGS. 7B and 7C structures comprise bores for containing and/orconducting fluids. The FIGS. 7B and 7C structures are also electricallyconductive. In some applications, the structures of FIGS. 7B, 7C and/orsimilar structures may be coupled to one another by extending onestructure into the bore of another, for example.

Circuit Boards

Electrically conductive elements may be fabricated on polymer-basedsubstrates or other substrates using the electrically conductive,thermosetting elastomeric materials described herein to fabricateflexible circuit boards similar to printed circuit boards (PCBs).Micromolding techniques similar to those described herein may be used tofabricate such flexible PCB-like circuit boards. FIGS. 12A-12E aremicrographs of example flexible PCB-like circuit boards fabricated fromsuch electrically conductive, thermosetting elastomeric materials usingsuch micromolding techniques. The particular composition used to providethe structures in FIGS. 12A-12E comprises a base polymer of PDMS and aconductive polymer additive of PEDOT in a 1:1 weight ratio and aparticulate filler of 80 nm-500 nm generally spherical silvernanoparticles with a concentration of about 30% weight. The substrate onwhich the FIG. 12A-12E PCB-like circuit boards are formed isnon-conductive PDMS.

The FIG. 12A micrograph shows a portion of a PCB-like circuit board. TheFIG. 12B micrograph is a magnified view of a portion of the FIG. 12APCB-like circuit board. The micrographs of FIGS. 12C and 12D showflexibility of the resultant PCB-like circuit board. FIG. 12E shows howelectrical contact can be made between conventional wires and a numberof ribbon cables on the PCB-like circuit board using conductive epoxy orthe like.

Some specific example embodiments of electrically conductive,thermosetting elastomeric materials and methods for their preparationare described below.

EXAMPLE 1

An example conductive thermosetting elastomeric material was prepared asfollows:

-   -   1 gram of generally spherical, 80-500 nm (average dimension)        silver nanoparticles was dispersed in 2 grams of heptane using        ultrasound waves for 30 seconds;    -   separately, 1.5 grams of PDMS elastomer was mixed with 3 grams        of Baytron™ FHC (PEDOT) conducting polymer until a uniform paste        was formed;    -   the above two mixtures were then manually stirred together for 3        minutes and then magnetically stirred at 1000-1500 rpm for 5        additional minutes.    -   a PDMS curing agent was then added, at a ratio of 1:10 with        respect to the PDMS elastomer, and the mixture was again        manually stirred for 3 minutes and magnetically stirred for 5        minutes;    -   the mixture (which was in liquid phase) was then either placed        in a vacuum or cooled to a temperature of 0° C. for 15 minutes        to remove air bubbles; and    -   finally, the mixture was heated at a temperature of 70° C. for 2        hours to cure into solid thick or thin films.

It will be apparent to one skilled in the art that the elastomericmaterial formed by this method could be hardened into various shapes bymeans of suitable molding or micromolding.

A 0.5 mm thick film of this example elastomeric material was measured tohave a sheet resistance of 0.05 Ω/square at NTP conditions.

EXAMPLE 2

Another example conductive thermosetting elastomeric material was formedas follows:

-   -   1 gram of 10 nm silver particles (which may or may not be PVC        coated in order to protect the particles from oxidizing when in        contact with air), 5 grams of heptane, 1.5 grams of PDMS        elastomer, and 3 grams of Baytron™ FHC (PEDOT) were combined in        a glass beaker and manually stirred for 5 minutes;    -   the mixture was then subjected to ultrasound waves for at least        60 seconds;    -   a PDMS curing agent was added, at a ratio of 1:10 with respect        to the PDMS elastomer, and the mixture was then manually stirred        for 4 minutes and magnetically stirred for 5 minutes;    -   the mixture (which was in liquid phase) was then either placed        in a vacuum or cooled to a temperature of 0° C. for 15 minutes        to remove air bubbles; and    -   finally, the mixture was heated at a temperature of 70° C. for 2        hours to cure into solid thick or thin films.

It will be apparent to one skilled in the art that the elastomericmaterial formed by this method could be hardened into various shapes bymeans of suitable molding or micromolding.

A 0.5 mm thick film of this example elastomeric material was measured tohave a sheet resistance of 0.01 Ω/square at NTP conditions.

EXAMPLE 3

A electrically conductive elastomer composition was prepared using abase polymer of PDMS and a particulate filler of multi-walled carbonnanotubes (MWCNT).

The MWCNT particles used were approximately 60-100 nm in diameter and5-15 μm in length. MWCNT have a relatively high aspect ratio (length toradius ratio) when compared to generally spherical particles and whencompared to metal-based nanorods, nanotubes and nanowires and cantherefore achieve the percolation threshold at lower concentrations.

Desired quantities of PDMS, MWCNT and PDMS-crosslinking agent werecarefully weighed (via digital balance) and six conductive elastomericsamples were prepared with varied MWCNT weight percentages of 0.80%, 1%,1.5%, 2%, 2.2% and 2.5%. The combination of the PDMS, MWCNT andconductive polymer additive was first manually stirred for approximately20 minutes and then oscillated by ultrasonic waves for approximately40-60 seconds. The PDMS crosslinking agent was the added to the mixturein a weight ratio of 1:10. It will be appreciated that other weightratios of PDMS-cross-linking agent could be used to achieve differentresult in the resultant elastomeric polymer (e.g. higher weight ratiosor curing agent could be used to achieve harder resultant materials andvice versa). Finally, the resultant mixture was manually stirred againfor approximately 20 minutes and again ultrasonically oscillated for 30minutes. After stifling, the mixture was placed into a vacuum chamber toremove air bubbles for 20 minutes prior to micromolding.

FIG. 8 is a plot depicting electrical conductivity (in siemens/meter(S/m)) of the Example 3 compositions versus weight percentage of MWCNT.

The Example 3 experiment was conducted without any conductiveelastomeric additive in the composition. Future embodiments, may alsocomprise conductive polymer additive (e.g. PEDOT) which is expected toincrease the resultant conductivity and decrease the particulate fillerweight percentage expected to achieve the percolation threshold.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof.

1. An electrically conductive, thermosetting elastomeric composition,the composition comprising: an initially substantially non-electricallyconductive, silicone-based thermosetting base polymer; a particulatefiller comprising electrically conductive particles; and an electricallyconductive polymer additive; wherein the non-electrically conductive,thermosetting base polymer, the particulate filler and the electricallyconductive polymer additive are mixed; wherein the electricallyconductive polymer additive comprises one or more of:ethylenedioxythiophene (EDOT); poly(3,4-ethylenedioxythiophene) (PEDOT);PEDOT doped with poly(styrenesulfonate) (PEDOT/PSS); polyaniline;poly(pyrrole); poly(acetylene); poly(thiophene); poly(p-phenylenesulfide); poly(para-phenylene vinylene) (PPV); polyindole; polypyrene;polycarbazole; polyazulene; polyazepine; and polynaphthalene; whereinthe weight ratio of the non-electrically conductive thermosetting basepolymer to the electrically conductive polymer additive is in a range of0.5:1 to 1.5:1.
 2. The composition according to claim 1 wherein thenon-electrically conductive, thermosetting base polymer comprises asilicone-based elastomer comprising one or more of: polydimethylsiloxane(PDMS) and a polysiloxane sealant formulation comprising from 85.0-100wt % of hydroxy-terminated polydimethylsiloxane, from 7.0-13.0 wt % ofamorphous fumed silica, and from 1.0-5.0 wt % of methyltriacetoxysilane.3. The composition according to claim 2 wherein the non-electricallyconductive, thermosetting base polymer comprises PDMS and thecomposition further comprises a curing agent suitable for curing PDMS.4. The composition according to claim 1 wherein the particulate fillercomprises one or more of: metal-based microparticles, metal-basednanoparticles, carbon-based microparticles and carbon-basednanoparticles.
 5. The composition according to claim 1 wherein theparticulate filler comprises metal-based nanoparticles of one or moreof: silver, gold, platinum, copper, nickel, aluminum, zinc, molybdenum,cadmium, scandium, titanium, vanadium, chromium, manganese, iron,cobalt, yttrium, zirconium, niobium, tantalum, tungsten, lead, indiumtin oxide, terfenol-D, manganin and constantan.
 6. The compositionaccording to claim 1 wherein the particulate filler comprisescarbon-based nanoparticles of one or more of: single walled carbonnanotubes, multi-walled carbon nanotubes, carbon nanorods, graphene,graphite and fullerene.
 7. The composition according to claim 1 whereinthe electrically conductive polymer additive comprises one or more of:poly(3,4-ethylenedioxythiophene) (PEDOT); and PEDOT doped withpoly(styrenesulfonate) (PEDOT/PSS).
 8. The composition according toclaim 1 wherein the particulate filler comprises a metal-basedparticulate filler and the composition comprises a particulate fillerconcentration of less than 30% weight.
 9. The composition according toclaim 8 wherein the aspect ratio of a maximum particle dimension to aminimum particle dimension of the particulate filler is greater than50:1.
 10. The composition according to claim 9 wherein the weight ratioof the non-electrically conductive thermosetting base polymer to theelectrically conductive polymer additive is in a range of 0.75:1 to1.25:1.
 11. The composition according to claim 8 wherein the weightratio of the non-electrically conductive thermosetting base polymer tothe electrically conductive polymer additive is in a range of 0.75:1 to1.25:1.
 12. The composition according to claim 1 wherein the particulatefiller comprises a metal-based particulate filler and the compositioncomprises a particulate filler concentration of less than 15% weight.13. The composition according to claim 12 wherein the aspect ratio of amaximum particle dimension to a minimum particle dimension of theparticulate filler is greater than 50:1.
 14. The composition accordingto claim 12 wherein the weight ratio of the non-electrically conductivethermosetting base polymer to the electrically conductive polymeradditive is in a range of 0.75:1 to 1.25:1.
 15. The compositionaccording to claim 1 wherein the particulate filler comprises acarbon-based particulate filler and the composition comprises aparticulate filler concentration of less than 5% weight.
 16. Thecomposition according to claim 15 wherein the aspect ratio of a maximumparticle dimension to a minimum particle dimension of the particulatefiller is greater than 250:1.
 17. The composition according to claim 1wherein the particulate filler comprises a carbon-based particulatefiller and the composition comprises a particulate filler concentrationof less than 2% weight.
 18. The composition according to claim 17wherein the aspect ratio of a maximum particle dimension to a minimumparticle dimension of the particulate filler is greater than 250:1. 19.The composition according to claim 1 wherein the composition is used tocreate a flexible, electrically conductive structure by: providing amold; introducing the composition to the mold; and allowing thecomposition to cure in the mold.
 20. The composition according to claim19 wherein using the composition to create the flexible, electricallyconductive structure comprises, after allowing the composition to curein the mold: applying a layer of non-electrically conductivethermosetting polymer against at least a portion of the curedcomposition in the mold; and allowing the layer of non-electricallyconductive thermosetting polymer to cure in contact with the portion ofthe cured composition.
 21. The composition according to claim 19 whereinusing the composition to create the flexible, electrically conductivestructure comprises, after allowing the composition to cure in the mold:(a) applying a layer of non-electrically conductive thermosettingpolymer against at least a portion of the cured composition in the mold;(b) allowing the layer of non-electrically conductive thermosettingpolymer to cure in contact with the portion of the cured composition;(c) introducing a second layer of the composition to the mold against atleast a portion of the cured non-electrically conductive thermosettingpolymer; (d) allowing the second layer of the composition to cure incontact with the portion of the cured electrically non-conductivethermosetting polymer.
 22. The composition according to claim 21 whereinusing the composition to create the flexible, electrically conductivestructure comprises, after allowing the composition to cure in the mold,repeating any of steps (a) through (d) to fabricate a structurecomprising multiple layers of flexible electrically conductive materialand multiple layers of flexible electrically non-conductive material.23. The composition according to claim 19 wherein: providing the moldcomprises iteratively fabricating a plurality of mold portions; andintroducing the composition to the mold comprises iterativelyintroducing a plurality of layers of the composition to the mold; andwherein iteratively introducing the plurality of layers of thecomposition to the mold comprises introducing a layer of the compositionto each mold portion and allowing the layer of the composition to cureprior to fabricating a subsequent mold portion.
 24. The compositionaccording to claim 23 wherein using the composition to create theflexible, electrically conductive structure comprises: iterativelyadding one or more layers of electrically non-conductive thermosettingpolymer to the structure, wherein iteratively adding one or more layersof electrically non-conductive thermosetting polymer to the structurecomprises adding a layer of electrically non-conductive, thermosettingpolymer against at least a portion of each layer of the compositionprior to fabricating a subsequent mold portion.
 25. An electricallyconductive, thermosetting elastomeric composition, the compositioncomprising: an initially substantially non-electrically conductive,thermosetting base polymer; a particulate filler comprising electricallyconductive metal-based microparticles or nanoparticles; wherein anaspect ratio of a maximum dimension to a minimum dimension of themetal-based particulate filler is greater than 50:1, and wherein theelectrically conductive polymer additive comprises one or more of:ethylenedioxythiophene (EDOT); poly(3,4-ethylenedioxythiophene) (PEDOT);PEDOT doped with poly(styrenesulfonate) (PEDOT/PSS); polyaniline;poly(pyrrole); poly(acetylene); poly(thiophene); poly(p-phenylenesulfide); poly(para-phenylene vinylene) (PPV); polyindole; polypyrene;polycarbazole; polyazulene; polyazepine; and polynaphthalene.